Automated blood pressure monitoring has rapidly become an accepted and, in many cases, essential aspect of human and veterinary treatment. Such monitors are now a conventional part of the patient environment in emergency rooms, intensive and critical care units, and in the operating theatre.
The so-called oscillometric method of measuring blood pressure is one of the most popular methods in commercially available systems. This method relies on measuring changes in arterial counterpressure, such as imposed by an inflatable cuff, which is controllably relaxed. In some cases the cuff bleed is continuous, and in others it is incremental, but in substantially all, a transducer monitors arterial counterpressure oscillations, and processing apparatus converts select parameters of these oscillations into blood pressure data. The units available from the assignee hereof under the trade name DINAMAP* Vital Signs Monitor provide an excellent example of automated blood pressure monitors employing the oscillometric methodology.
A prime aspect of accurate automated monitoring is the discrimination of true counterpressure data from artifact data. Such artifact data may result from physiological events such as patient movement or shock to the cuff through external contact, or may result from internally generated artifacts, such as noise or other effects produced by system componentry.
One class of artifact rejection techniques is exemplified by those taught in U.S. Pat. Nos. 4,349,034 and 4,360,029 to M. Ramsey, III, commonly assigned herewith. The Ramsey patents disclose an incremental deflation apparatus wherein, at each pressure level, plural counterpressure oscillatory complexes produced by heartbeats (referred to herein simply as "complexes") are detected, and select parameters such as peak height and time rate of change of successive samples and successive series of samples are evaluated relative to specified artifact discrimination criteria. See also U.S. Pat. Nos. 3,903,872; 4,009,709; 4,074,711; 4,154,238; 4,174,707; 4,367,751 to Link.
It will be apparent, then, that even those systems employing very elaborate artifact rejection algorithms to process the parameters of the oscillatory complexes are subject to error if the complexes themselves are inadequately or inaccurately sensed in the first instance. In particular, it is noted that the counterpressure complexes constitute relatively low level, noisy signals superimposed on the much larger, sometimes poorly regulated cuff pressure baseline. Accordingly, it is conventional first to subject the signals to band pass filters which tend to pass the complex but to block noise on the high frequency end, and the base cuff pressure and slower variations thereof at the low end. It has been found, however, that this very filtering, and particularly the high pass sections thereof, can contribute inaccuracy to the complex detection and measurement process. In such a filter, which can be simply modeled as a single section RC high pass network, the input coupling capacitor acts as a memory element. That is, signals in the near or distant past have some effect on signals generated at any time, based upon the amplitude of such signals (which proportionally add charge to the capacitor when they occur) and the RC time constant of the network (which establishes the rate of exponential discharge of the capacitor). Accordingly, it is possible at any given time for the actual counterpressure signal, when applied to the high pass input filter, to be "combined" with remnants of prior signals as represented by instantaneous charge on the capacitor, and thus to be erroneously interpreted by subsequent processing circuitry.
It is an object of the present invention to eliminate the untoward effects on the complex detection process imposed by operation of input filtering componentry.